Glutathione peroxidase 4 and vitamin E control reticulocyte maturation, stress erythropoiesis and iron homeostasis.

Glutathione peroxidase 4 (GPX4) is unique as it is the only enzyme that can prevent detrimental lipid peroxidation in vivo by reducing lipid peroxides to the respective alcohols thereby stabilizing oxidation products of unsaturated fatty acids. During reticulocyte maturation, lipid peroxidation mediated by 15-lipoxygenase in humans and rabbits and by 12/15-lipoxygenase (ALOX15) in mice was considered the initiating event for the elimination of mitochondria but is now known to occur through mitophagy. Yet, genetic ablation of the Alox15 gene in mice failed to provide evidence for this hypothesis. We designed a different genetic approach to tackle this open conundrum. Since either other lipoxygenases or non-enzymatic autooxidative mechanisms may compensate for the loss of Alox15, we asked whether ablation of Gpx4 in the hematopoietic system would result in the perturbation of reticulocyte maturation. Quantitative assessment of erythropoiesis indices in the blood, bone marrow (BM) and spleen of chimeric mice with Gpx4 ablated in hematopoietic cells revealed anemia with an increase in the fraction of erythroid precursor cells and reticulocytes. Additional dietary vitamin E depletion strongly aggravated the anemic phenotype. Despite strong extramedullary erythropoiesis reticulocytes failed to mature and accumulated large autophagosomes with engulfed mitochondria. Gpx4-deficiency in hematopoietic cells led to systemic hepatic iron overload and simultaneous severe iron demand in the erythroid system. Despite extremely high erythropoietin and erythroferrone levels in the plasma, hepcidin expression remained unchanged. Conclusively, perturbed reticulocyte maturation in response to Gpx4 loss in hematopoietic cells thus causes ineffective erythropoiesis, a phenotype partially masked by dietary vitamin E supplementation.

Glutathione peroxidase 4 prevents necroptosis in mouse erythroid precursors.

Maintaining cellular redox balance is vital for cell survival and tissue homoeostasis because imbalanced production of reactive oxygen species (ROS) may lead to oxidative stress and cell death. The antioxidant enzyme glutathione peroxidase 4 (Gpx4) is a key regulator of oxidative stress-induced cell death. We show that mice with deletion of Gpx4 in hematopoietic cells develop anemia and that Gpx4 is essential for preventing receptor-interacting protein 3 (RIP3)-dependent necroptosis in erythroid precursor cells. Absence of Gpx4 leads to functional inactivation of caspase 8 by glutathionylation, resulting in necroptosis, which occurs independently of tumor necrosis factor α activation. Although genetic ablation of Rip3 normalizes reticulocyte maturation and prevents anemia, ROS accumulation and lipid peroxidation in Gpx4-deficient cells remain high. Our results demonstrate that ROS and lipid hydroperoxides function as not-yet-recognized unconventional upstream signaling activators of RIP3-dependent necroptosis.

The Bcl10-Malt1 complex segregates Fc epsilon RI-mediated nuclear factor kappa B activation and cytokine production from mast cell degranulation.

Mast cells are pivotal effector cells in IgE-mediated allergic inflammatory diseases. Central for mast cell activation are signals from the IgE receptor FcepsilonRI, which induce cell degranulation with the release of preformed mediators and de novo synthesis of proinflammatory leukotrienes and cytokines. How these individual mast cell responses are differentially controlled is still unresolved. We identify B cell lymphoma 10 (Bcl10) and mucosa-associated lymphoid tissue 1 (Malt1) as novel key regulators of mast cell signaling. Mice deficient for either protein display severely impaired IgE-dependent late phase anaphylactic reactions. Mast cells from these animals neither activate nuclear factor kappaB (NF-kappaB) nor produce tumor necrosis factor alpha or interleukin 6 upon FcepsilonRI ligation even though proximal signaling, degranulation, and leukotriene secretion are normal. Thus, Bcl10 and Malt1 are essential positive mediators of FcepsilonRI-dependent mast cell activation that selectively uncouple NF-kappaB-induced proinflammatory cytokine production from degranulation and leukotriene synthesis.

Direct crosstalk between mast cell-TNF and TNFR1-expressing endothelia mediates local tissue inflammation.

Signaling through tumor necrosis factor receptor 1 (TNFR1) controls bacterial infections and the induction of inflammatory Th1 cell-mediated autoimmune diseases. By dissecting Th1 cell-mediated delayed-type hypersensitivity responses (DTHRs) into single steps, we localized a central defect to the missing TNFR1 expression by endothelial cells (ECs). Adoptive transfer and mast cell knockin experiments into Kit(W)/Kit(W-v), TNF(-/-), and TNFR1(-/-) mice showed that the signaling defect exclusively affects mast cell-EC interactions but not T cells or antigen-presenting cells. As a consequence, TNFR1(-/-) mice had strongly reduced mRNA and protein expression of P-selectin, E-selectin, ICAM-1, and VCAM-1 during DTHR elicitation. In consequence, intravital fluorescence microscopy revealed up to 80% reduction of leukocyte rolling and firm adhesion in TNFR1(-/-) mice. As substitution of TNF(-/-) mice with TNF-producing mast cells fully restored DTHR in these mice, signaling of mast cell-derived TNF through TNFR1-expressing ECs is essential for the recruitment of leukocytes into sites of inflammation.

Health effects of ambient particulate matter--biological mechanisms and inflammatory responses to in vitro and in vivo particle exposures.

In this article, we review and analyze different modes of exposure to ultrafine particles in order to assess particle-induced inflammatory responses and the underlying mechanisms in vitro and in vivo. Based on results from monocytic cells cultured under submerged conditions, we discuss (1) the impact of particle properties such as surface area and oxidative potential on lipid metabolism as a highly sensitive regulatory pathway and (2) the interference of diesel exhaust particles with toll-like receptor-mediated inflammatory responses. Furthermore, new developments of air-liquid interface exposure used as an alternative approach to simulate cell particle interactions are presented. In addition to the in vitro approaches, animal exposure studies are described that apply selected mouse models to elucidate potential allergic and inflammatory pulmonary responses and mast-cell-related mechanisms after particle exposure. Long-term inhalation of ultrafine particles might lead to irreversible changes in lung structure and function. Clinical studies addressing the characteristics of inflammatory airway cells are a promising approach to understand underlying pathophysiological mechanisms in chronic obstructive pulmonary disease. Finally, a potential outcome of human particle exposure is chronic cough in children. Here, discrimination between asthmatic and nonasthmatic cough by means of immunological parameters appears to be an important step toward improving diagnosis and therapy.

Classical and alternative pathways of mast cell activation.

It has long since been recognized that mast cells are critical effectors of anaphylactic reactions, and the existence of these potentially hazardous cells has solely been justified due to their beneficial role in some infections with extracellular parasites. A novel understanding of mast cells as sentinels of the immune system has been made possible by taking advantage of mast cell-deficient mice in order to study the roles of mast cells in vivo and by detailed analyses of mast cell activation in vitro. Collectively, these experiments have revealed a variety of IgE-independent stimuli, which lead to the activation of mast cells as crucial initiators of an inflammatory response. Besides their effector function, a variety of studies reviewed herein point towards important regulatory roles for mast cells, especially regarding their interaction with other cell types.

Mast cells and inflammation.

Mast cells have long been recognized as potent producers of a large panel of biologically highly active mediators such as biogenic amines, arachidonic acid metabolites, cytokines and chemokines, but most of their biological functions have been elusive and speculative. By taking advantage of mast cell-deficient mice, the role of mast cells in a variety of experimental settings can now be studied in detail and such approaches have dramatically altered and enlarged our knowledge about mast cell biology and function. Herein we will focus on the role of mast cells in inflammatory reactions of diverse origin, such as delayed type hypersensitivity, atopy, immune complex-mediated inflammation and innate immune responses. From the current standpoint, there is no doubt that the most outstanding and beneficial feature of mast cells is their recently discovered ability to induce a life-saving inflammatory response rapidly upon encountering microbes and microbial constituents. Nevertheless, the picture is also emerging that mast cells are deeply involved in the induction and maintenance of a variety of severe allergic and autoimmune diseases. However, a deeper understanding of their activation and immune-modulatory capacity might open a new window for the development of curative strategies.

Targeted mast cell silencing protects against joint destruction and angiogenesis in experimental arthritis in mice.

OBJECTIVE: Induction of arthritis with autoantibodies against glucose-6-phosphate isomerase (GPI) is entirely independent of T cells and B cells but is strictly dependent on the presence of mast cells. Here, we used this disease model to analyze whether exclusive intraarticular mast cell reconstitution is sufficient for disease induction and whether targeted mast cell silencing can prevent neoangiogenesis and joint destruction, 2 hallmarks of rheumatoid arthritis. METHODS: Ankle swelling and clinical index scores were determined after injection of either K/BxN mouse-derived serum or control serum in wild-type Kit(+)/Kit(+) mice, congenic mast cell-deficient Kit(W)/Kit(W-v) mice, or mast cell-deficient Kit(W)/Kit(W-v) mice reconstituted with mast cells, either by intraperitoneal or selective intraarticular injection. Angiogenesis was quantified in vivo by measuring activated alphavbeta3 integrin using (18)F-galacto-RGD and positron emission tomography. In addition, staining of joint tissue with hematoxylin and eosin, Giemsa, beta3, and alpha-actin was performed. The effect of mast cell stabilization by treatment with cromolyn or salbutamol was investigated in C57BL/6 or BALB/c mice. RESULTS: Comparing wild-type mice, mast cell-deficient Kit(W)/Kit(W-v) mice, and mast cell-reconstituted Kit(W)/Kit(W-v) mice, we first showed that intraarticular and intraperitoneal mast cell engraftment fully restores susceptibility to antibody-induced arthritis, angiogenesis, and alphavbeta3 integrin activation. Importantly, selective mast cell silencing with either salbutamol or cromolyn prevented alphavbeta3 integrin activation, angiogenesis, and joint destruction. CONCLUSION: Mast cell engraftment fully restores susceptibility to alphavbeta3 integrin activation, angiogenesis, and joint destruction in GPI antibody-induced arthritis. Importantly, selective mast cell stabilization prevents alphavbeta3 integrin activation, angiogenesis, and joint destruction.

Mast cells and endothelin-1: a life-saving biological liaison?

Over a decade ago endothelin-1 (ET-1) was described as belonging to the assortment of mast cell-derived and mast cell-bound cytokines. Mast cell subtype- and environment-dependent, ET-1 acts as a potent non-immunological mast-cell activator, which leads to degranulation and mediator release. The biological significance of this phenomenon remained obscure; however, a recent report sheds new light on this delicate mast cell-ET-1 connection. The authors of this report use mast cell-deficient mice to provide convincing evidence that mast cells counteract the toxicity induced by high concentrations of ET-1, and thereby promote homeostasis.

The streptococcal exotoxin streptolysin O activates mast cells to produce tumor necrosis factor alpha by p38 mitogen-activated protein kinase- and protein kinase C-dependent pathways.

Streptolysin O (SLO), a major virulence factor of pyogenic streptococci, binds to cholesterol in the membranes of eukaryotic cells and oligomerizes to form large transmembrane pores. While high toxin doses are rapidly cytocidal, low doses are tolerated because a limited number of lesions can be resealed. Here, we report that at sublethal doses, SLO activates primary murine bone marrow-derived mast cells to degranulate and to rapidly induce or enhance the production of several cytokine mRNAs, including tumor necrosis factor alpha (TNF-alpha). Mast cell-derived TNF-alpha plays an important protective role in murine models of acute inflammation, and the production of this cytokine was analyzed in more detail. Release of biologically active TNF-alpha peaked approximately 4 h after stimulation with SLO. Production of TNF-alpha was blunted upon depletion of protein kinase C by pretreatment of the cells with phorbol-12 myristate-13 acetate. Transient permeabilization of mast cells with SLO also led to the activation of the stress-activated protein kinases p38 mitogen-activated protein (MAP) kinase and c-jun N-terminal kinase (JNK), and inhibition of p38 MAP kinase markedly reduced production of TNF-alpha. In contrast, secretion of preformed granule constituents triggered by membrane permeabilization was not dependent on p38 MAP kinase or on protein kinase C. Thus, transcriptional activation of mast cells following transient permeabilization might contribute to host defense against infections via the beneficial effects of TNF-alpha. However, hyperstimulation of mast cells might also lead to overproduction of TNF-alpha, which would then promote the development of toxic streptococcal syndromes.